This invention relates to a color image sensor with color filters and optical diffusion members and, in particular, to a color image sensor adapted for a copying machine and facsimile equipment and adapted to subject a document image to a photoelectric conversion so as to form an image.
With the recent rapid increase in computer-assisted office automation, high-speed, high-resolution, and compact-sized copying machines, facsimile equipment, etc. have been developed as contact type image sensors. The term "contact type image sensor" means, in this case, an image sensor in which, in place of a conventional scale-down optical system equipped with a spherical lens, a light-convergent optical lens (Selfoc lens) is employed as an optical system, with an image sensor leading to a document plane, and in which a document image equal in size to the original is formed on the image sensing elements.
FIG. 5 shows contact type image sensor (CCD device) 21, which comprises a linear array of photoelectric conversion elements 22a, 22b, 22c. Color filters, for example, red (R), green (G), and blue (B) color filters 23a, 23b, 23c are arranged in a cyclic fashion on photoelectric conversion elements 22a, 22b, 22c. Each set consisting of an (R), a (G), and a (B) filter, along with their respective photoelectric conversion elements 22a, 22b, 22c constitute a picture element 24. Since image sensor 21 has the double advantage of achieving a high integration density and high-speed operation of picture elements 24 through the use of the integral-circuit technique, a color image on a document can be quickly read out through the reading of a respective image corresponding to each picture element, from the document by the aforementioned tricolor (R, G, B) filter.
In the aforementioned image sensor 21, however, a light shield 25, formed of, for example, an aluminum-evaporated film, is located at the boundary of each of photoelectric conversion elements 22a, 22b, 22c and at the boundary of each of color filters 23a, 23b, 23c. As a result, respective photoelectric elements 22a, 22b, 22c are independently and individually driven upon receipt of incident light. Where, therefore, an image is formed whose size is, for example, equal to or smaller than that of one photoelectric conversion element, there is the possibility that, due to the resolution of respective color filters 23a, 23b, 23c, a variation in the brightness level of the image may be read out as a variation in the hue and saturation levels of the image.
To put it another way, where the image on the document is smaller than one picture element (24) corresponding to the three photoelectric conversion elements on the image sensor, it cannot be imparted as an uniform image formation to the three photoelectric conversion elements, and thus the output balance among the respective photoelectric conversion elements is disturbed with respect to picture element 24, with the result that a true color image signal corresponding to the image on the document, that is, accurate color data on the color document, sometimes cannot be obtained.
Where, in particular, a monochromatic image is read out, it is not possible in the prior art to accurately read out the actual brightness, hue, and saturation levels as color image data, and the density of the document image.